I think solipsist’s point isn’t that they have an excuse but that they have a reason—being monoglot hurts them less than it would if they were e.g. on the European continent, so monoglossy (or whatever the right word is) isn’t necessarily silly for them.
[EDITED to add:] Disappointingly, OED suggests that the right word is just “monoglottism”.
I think solipsist’s point isn’t that they have an excuse but that they have a reason—being monoglot hurts them less than it would if they were e.g. on the European continent, so monoglossy (or whatever the right word is) isn’t necessarily silly for them.
[EDITED to add:] Disappointingly, OED suggests that the right word is just “monoglottism”.